1. Field of the Invention
This invention relates to automatic drive belt tensioning apparatus and, more particularly, to an apparatus of the type having a pivotable tensioner arm with an idler pulley thereon which is borne against an unsupported portion of a drive belt to create a predetermined tension thereon.
2. Background Art
It is conventional to use a belt tensioning apparatus on drive belt systems such as those in the automobile industry. The automobile systems are generally characterized as serpentine belt systems. In such systems, an endless belt is trained in a circuitous path about a plurality of accessories, such as pumps, alternators, etc. The belt frictionally drives pulleys associated with shafts on the accessories. For the accessories to operate consistently and effectively, it is necessary for the frictional drive force developed on the accessory pulleys to be relatively constant. This is conventionally accomplished by producing a predetermined tension on the belt through a belt tensioning apparatus.
Typically, such tensioners employ hydraulic or pneumatic mechanisms, rubber and a steel spring or resin, etc. to impart a desired tension on the belt through compression, tension, bending, or twisting.
One objective of automobile designers is to maximize the passenger compartment space and minimize the compartment space for the engine With the premium on space in the engine compartment, it is important that all engine components, including the belt tensioner, be made as small as possible without compromising the operation thereof.
As a rule, prior art tensioners have had a very substantial dimension both axially and radially with respect to the pivot axis for the tensioner arm. For example, U.S. Pat. No. 4,473,362, to Thomey et al, shows a belt tensioning apparatus with a coil spring 44 that exerts a torsional bias on a tensioner arm 78 to thereby bear an idler pulley 42 on the end of the arm 78 against a drive belt. Due to the large torsional force that the coil spring 44 must develop, the wire from which the spring 44 is formed is heavy gauge. The required torque that must be produced also necessitates a large number of coil turns with relatively large radii. The spring 44 in Thomey et al must also have considerable strength axially thereof to perform a biasing function as part of a damping mechanism. The result is that the overall dimensions of the belt tensioning apparatus become quite large to accommodate the spring 44.
One prior art inventor, Kraft, in U.S. Pat. No. 4,285,676 resorts to the use of multiple springs to produce the desired forces. The resulting dimensions of the tensioning apparatus, particularly radially of the tensioner arm pivot axis, become quite large.
The space problems are further aggravated by the provision in certain prior art structures, such as that in the above Thomey et al patent, of damping mechanisms to control pivoting of the tensioner arm. This damping feature is generally incorporated to prevent tensioner arm backlash and vibrations that may cause a resonant condition in the drive belt and/or tensioning apparatus, as may occur with abrupt acceleration in automobiles.
One such damping mechanism is shown in U.S. Pat. No. 4,362,525, to Sproul. Sproul has a damping mechanism 58 that is spaced a considerable distance away from the structure for biasing the tensioner arm 25 and associated idler pulley 35. This necessitates the provision of a large mounting bracket 10 which competes with other engine accessories for space
Another known damping mechanism is that in U.S. Pat. No. 4,661,087, to Henderson, which employs a piston or blade to be operated in a liquid to create a damping force. A considerable amount of space is required for this mechanism as well.